Case syncretism is the paradigmatic synthesis of cases (multiple case functions are marked by the same form).[1] Nominal case syncretism is the phenomenon in which an inflectional form (case marking) is used to express two or more case functions. Case functions are determined to exist when nominal inflection occurs and the given function is marked by a specific inflectional form of at least one nominal category or subcategory[2].

Types:

NoCase: The language does not use case marking.

NoSyncCase: Nouns show case marking, and cases are never syncretic.

SyncCoreCase: Nouns show case marking, and syncretism occurs only in core cases[3].

SyncNonCoreCase: Nouns show case marking, with syncretism only occurring in non-core cases.[4]

SyncCase: Nouns show case marking, with syncretism occurring in both core and non-core cases.

[1] Compare to exponence, which is the syntagmatic synthesis of cases.

[2] Case syncretism does not occur, therefore, in the following cases: (1) if a language only shows case marking on particular pronouns, as in English (this phenomenon can be better described as the lack of declension of other nominals, not case syncretism); (2) if two case functions are marked by the same form on every nominal (this is not syncretism but one case with more than one semantic function). In contrast, if the pronominal or nominal paradigm (or any subtype of theirs) contains at least three case functions, any paradigmatic overlaps are considered examples of syncretism. In Russian, for example, inanimate nouns generally display the same form in the nominative and accusative cases; this is considered an example of syncretism because distinct forms across the two cases can be found in a significant portion of feminine nouns. Similarly, Finnish nouns, based on the pronominal paradigm, display syncretism in their nominative and genitive cases, inasmuch as the nominative or genitive case can express the accusative function in addition to their basic function (the choice between the two being determined by the syntactic and/or semantic context).

[3] In nominative languages, core case refers to the cases marking the subject and the object (nominative and accusative). In other systems, it refers to equivalent cases that express the meaning or function of transitive verbs, such as ergative and absolutive cases for ergative languages.

[4] This possibility exists only in theory; no examples of it have yet been found.